The author previously reported that the serum calcium (Ca) and inorganic phosphorus (P) levels were gradually decreased with advance in age in regularly menstruating women, but that they were rapidly increased soon after the menopause. In this experiment the changes of the serum Ca and P levels in postmenopausal women were studied more precisely in 410 peri- or post-menopausal subjects and 534 controls with regular menses. They were classified into 12 groups according to age or period since last menses. The clavicular cortical thickness was also measured as an index of bone resorption. Blood samples were taken between 9-11 A.M. after an overnight fast and the separated sera were applied to a Technicon Autoanalyzer SMA 12/60 and analyzed. The mean levels of Ca and P in serum were both increased following the menopause and reached a plateau in Groups F-3 (12-24 months since last menses) and G (2-5 years since last menses). In Group G (5-10 years since last menses) both the levels were slightly declined. Then a dissociation was observed in Groups I-1 and I-2. This was a successive decrease of Ca and a re-increase of P. This dissociation might be explained partly by the physiologic fall of parathyroid function. Furthermore the clavicular cortical thickness was measured with a micrometer in the middle of the clavicle on the chest X-ray film of the same subjects and the clavicular score was computed as Cortical width X 100/Whole width. The score was gradually but steadily reduced with the lapse of time after the menopause and in Group I-2 it was lowered to approximately 45% of that in the premenopausal groups. From these results it is certain that the decline of estrogen secretion led to the elevation of the serum Ca and P levels and subsequently to the reduction in the clavicular score.